The invention relates to a method and apparatus for the generation of hot gases with temperatures of above 1,600.degree. K. for the chemical and metallurgical industry.
The invention has the purpose of rendering the heating and/or production of the gases of any desired composition, including gaseous mixtures, more economical in an electric-arc furnace.
The gases to be heated up can be of varying composition; it is possible to charge air as well as gaseous mixtures of carbon monoxide (CO) and hydrogen (H.sub.2). Besides CO and H.sub.2, the gaseous mixture can also consist of carbon-containing components reacting with an oxidizing medium added in suitable quantities, such as, for example, CO.sub.2, H.sub.2 O, etc., at the prevailing temperatures and pressures, thus yielding a gaseous mixture containing predominantly CO and H.sub.2. In this case, part of the energy fed to the arc is used for promoting the chemical reaction.
Such gaseous mixtures are utilized in chemical industry as synthesis gas for the manufacture of various organic compounds. In the production of metal from ores, these mixtures serve as a reduction gas.
In the classical blast furnace method, the heat for the heating process is generated in the blast furnace by burning coke with preheated air. The exhaust gas of the blast furnace is burned and the sensible heat is transferred to the air to be preheated in the blast heaters. During this step, temperatures are reached of 1,200.degree.-1,500.degree. K.; a technical limit lies at about 1,600.degree. K. In contrast thereto, in the recently developed direct reducing processes, the heat for the heating-up step is produced essentially outside of the reducing process and transferred to the reduction gases. Even relatively small metallurgical plants can be operated by using this method. In this connection, utilization of electric-arc furnaces for the manufacture and/or heating up of the reduction gases may be advantageous for economical reasons. In this connection, gas temperatures can be attained of up to 3,500.degree. K. under pressures of up to 10 bar.
Heating of certain gases, with or without chemical reactions, in an electric-arc furnace is conventional; for example, acetylene is produced from hydrocarbons on a large industrial scale in an electric-arc furnace (Hydrocarbon Processing and Petroleum Refining, 1962 Vol. 4, No. 6, 159 et seq.). In the metal-smelting industry, gaseous mixtures of CO and H.sub.2 are produced and/or heated in an electric-arc furnace; also air can be heated up in an electric-arc furnace (German Pat. No. 2,413,580, corresponding to British Pat. No. 1,473,942). For this purpose, varying arc arrangements are utilized, such as, for example, gas-vortex stabilized arcs, magnetic-field stabilized arcs, and high-current arcs, as well as combinations of these arrangements. In gas-vortex stabilized arcs, the cathode and/or anode can be of rod or plate shape (U.S. Pat. No. 4,002,466). The disadvantage of this method resides in that the striking point of the arc is too greatly fixed on one spot of the electrode, leading to short operating lifetimes (50-100 hours) of the electrode--especially at high electrical power. Furthermore, this method can only be carried out with gaseous hydrocarbons and under pressures of below 1.5 bar (absolute) in a stable fashion.
In magnetic-field stabilized arcs, the electrodes consist, at least in the zone of the axial magnetic field, of nonmagnetizable metals, for example, copper or steel alloys (DOS No. 2,107,824 and British Pat. No. 1,351,626). The strong burnout of these electrodes can cause considerable costs for the electrodes and for the removal of the burnout products. For this reason, the gas-vortex stabilized arc furnaces with low-burnout and inexpensive electrodes have become popular for electric-arc furnaces having powers of above 6 MW. It is desirable to utilize this electric-arc furnace also for the heating up and/or manufacture of reduction gases or synthesis gases, as well as for heating up air under relatively high pressures.
In the conventional, gas-vortex stabilized high-voltage arc furnace, as utilized on a large industrial scale for acetylene manufacture, the strike points of the arc lie on large-area electrodes; thereby, an only minor burn-off occurs, resulting in lifetimes for the electrodes of 1,000 hours and more. However, this known arc arrangement can be operated in a stable fashion only with gaseous hydrocarbons and under pressures of below 1.5 bar (absolute). Changes in the composition of the gaseous mixture lead to considerable alterations in electric field strength in the longitudinal direction, affecting the arc length and, thus, the amount of gas required for vortex stabilization. At pressures of above 1.5 bar (absolute), the rotational flow of the gas, necessary for stabilizing the arc, is considerably altered. These influential variables are critical in a gas-vortex stabilized high-voltage arc having a ratio of voltage to current larger than or equal to 4 volt/ampere.
Thus, the problem presents itself of designing the gas-vortex stabilized high-voltage arc, operated with direct current, in an appropriate way and of providing process conditions making it possible to operate this furnace in a stable fashion with air, with oxygen-enriched air, or with gaseous mixures of CO, H.sub.2, or gaseous hydrocarbons, as well as with CO.sub.2 and/or steam under pressures of above 1.5 bar (absolute).